Radio signaling system



. 8, 1936. L. E. BARTON RADIO SIGNALINGSYSTEM 3 She'ets-Shet l Original Filed-Jan. 15, 1952 IN VENTOR 1 .09 E..Barton HIS TTHNEY .n.9 Nw. sooom Da; s, 1936. L. E. BARTON Y l2,063,290'

RADIO SIGNLING SYSTEM Original Filed Jan. l5, 1932 3 Sheets-Sheet 2 muuu - INvEN'roR Lo larton.

Dec. 8, 1936.

L. E. BARTON 'RADIO SIGNALING SYSTEM Original Filed Jan. l5, 1932 3 Sheets-Sheet 3 Patented Dec. 8, 1936 PATENT oFFicE RADIO SIGNALING SYSTEM ,Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Original application January 15, 1932, Serial No. 586,874. Divided and this application May 31,

1934, Serial No. 728,267.

Claims.

The present invention relates to radio signaling systems and more particularly to systems of that character embodying electric discharge devices as audio frequency amplifiers and modulators,

being a, division of my copending application frequency amplifier systems has steadily in.

creased, in both radio receiving and transmitting apparatus.

In radio receiving apparatus, as is well known,

,'the demand for higher p'ower output occurs chiefly in connection with the output stage of the audio frequency amplifier, while in transmitting apparatus it occurs both in connection with the power amplifier and in connection with the modulator, used in conjunction with the power amplifier, to apply the modulating or audio frequency signal to the carrier wave.

This demand for higher audio frequency power has heretofore been met chiefly by the development of tubes having a higher plate or anode dissipation and a lower plate or anode resistance, and the use of these or available standard tubes in balanced or .push-pull relation and in multiple, both in radio receiving and amplifying apparatus and in transmitting apparatus. This expedient has the disadvantage that it involves a multiplicity of tubes and associated apparatus, with corresponding complication and cost. 'I'his is particularly true in connection with transmitters arranged for substantially 100 per cent modulation and having appreciable power output,for the reason that the power requirements for a suitable modulator therefor is of a corresponding magnitude and cost.

When the antenna or output current of a transmitter is caused to fluctuate according to a modulating wave of any shape, the average antenna current may remain the same, but the effective value of the current increases, therefore, increasing the average power input to the antenna. 'I'his increase in antenna power must be supplied (a) by the modulators employed, (b) by an equivaient increase in power, (c) by an increase in the plate current to the power amplifier tubes, or, (d) by an increase in the average efficiency of the power amplifier tubes.

It is generally conceded that the increased power may best be obtained by method (a) above, in supplying the increased power in the form of audio frequency power to the anode or plate cir- In Mexico January cuit of the radio frequency power amplifier.

However, this has heretofore involved the use'oftubes of higher rating, adapted to carry the larger modulator power output. Therefore, in meeting such increased power requirements, modulators heretofore and at present may often include a plurality of high power tubes connected in balanced and in multiple relation, with attendant high cost of operation as well as initial cost.

It is, therefore, an object of the present invention to provide an improved modulator adapted to economically supply relatively large amounts of audio frequency power with electric discharge devices of normal power rating and without employing an excessively large number of such devices as has heretofore been necessary.

It is well recognized that 100 per cent modulation is the most economical degree of modulation.

This is for one reason, among others, that the higher percentage modulation permits the use of a transmitter of lower power for the production of the same signal strength at a given distance from the transmitter. Furthermore, the higher modulated signal is subject to much less interference.

For 100 per cent modulation it will be seen that in a high power transmitter employing a class C amplifier, a relatively large amount of audio frequency power is required from the modulator. It is a common practice at present to supply the modulating power to such amplifiers by a modulator having a plurality of electric discharge amplifiers connected in multiple or balanced relation, the number of such devices being in any case relaitvely large and depending upon the amount of power required from the modulator. As has been shown heretofore, the lamount of power required for modulation purposes may be in the order of the power output of the output amplifier.

The present invention relates more particularly to the class ,B type of amplifier as the output stage of an audio frequency amplifier unit for signaling systems, and has for a further object to provide an improved audio frequency electric discharge amplifier or modulator of that type from which a higher undistorted output power may be obtained at audio frequencies than has heretofore been obtainable by known means or arrangement of electric discharge devices, and which is adapted to supply large amounts of audio frequency power sufficient to modulate a class C or a class B output amplifier for a transmitter of relatively high power, to any desired degree of modulation.

In the class B of amplifier, the outputor -power tubes are biased to essentially anode curfair degree. However, like the class A amplifier, this type requires relatively low audio frequency power from the signal source to operate it, and the input system thereto may be the usual relatively high resistance type. An amplifier of the class B type, however, does not in itself satisfy all requirements, and must be of the type described and claimed in the application of which this is a division. As has been pointed out this requires that the amplifier tubes be arranged in balanced relation to each other and that the input and output impedances of the circuits connected therewith must be lower than the lowest internal operating impedances of the tubes whereby the tubes may be operated to the limit of their space charge or emission.

It is, therefore, a further object of this invention to provide an improved audio frequency amplifier arranged to operate the electric discharge devices or tubes therein in such a manner and to such limits that they may supply an audio i a relatively high frequency output relatively higher than the normal output of the same electric discharge devices or tubes with the same anode voltage, with a lower averagey anode dissipation, 'and with no serious effects upon their life. Thus, in accordance with the invention, the usual requirements for higher plate or anode dissipation and higher anode voltages, both of which are costly in tube construction and in the matter of the supply of operating potentials, are entirely obviated.

In accordance with the invention there may be provided an audio frequency amplifier by which audio frequency output, for example of five to ten times the usual or normal 'output, may be obtained without increasing the size, rating, or number of electric discharge devices employedtherein, and without increasing the anode potential or anode dissipation.

An amplifier of this type may be used as a source of high audio frequency power for plate modulation of a transmitter as hereinbefore described, and may be applied to any signaling system requiring a relatively high audio frequency output, with a minimum of equipment.

Because of the class B type of operation, a varying load is placed upon the anode power supply for an improved amplifier and modulator embodying the invention; and accordingly it is a further object of' the invention to provide means in connection with the source of anode or plate current supply, whereby fluctuations in load are prevented from appreciably affecting the voltage regulation of the source.

The invention will be better understood from r the following description when taken in connecdrawings.

' relatively low power output in the driver stage,

which may beof the class A type for example, the output stage may be driven substantially to the limit of the emission or space charge of the electric discharge devices or tubes therein for maximum'audio frequency power output. For a given power output, the latter tubes also may be.

of normal power rating as will be seen hereinafter.

The amplifier I2 receives audio frequency signal voltage from any suitable source such as input terminals I3 which are connected withit through a suitable input or grid circuit Il, whereby audio frequency signal' potentials applied to said terminals or to the input of the amplifier, are amplified by the device I2 and through its output or anode circuit indicated at I5, are applied to the output stage I0.

The output amplifier stage consists essentially of two tubes I6 and I1 connected in push-pull or balanced relation between push-pull input and output coupling devices I8 and I9 respectively, and biased for class B operation, as indicated. The grid bias supply indicated is preferably of relatively low direct current resistance. Theoutput stage is provided with a suitable anode voltage or plate supply source as `indicated in the The anode voltage supply source should have good voltage regulation under wide load variations, as it should be understood that the load current increases with an increase in the amplitude of applied signal voltages in this type of amplifier.

The output stage is provided with an input circuit or system, including the coupling device I8 and the output circuit I5 of the driver stage, having a -relatively low impedance in itself, whereby the signal voltage is vpermitted to carry the grids of the tubes I6 and I1 therein, far into the positive range of operation, without a reduction of the available signal voltage and hence distortion, because of the impedance drop in such input circuit or system, and without excessively loading the driver stage.

The driver or first stage of the amplifier is preferably but not necessarily, transformer coupled to the output stage. In the present example, device I8 is a transformer having a. step down ratio from a primary winding 20 in the anode circuit I5, to each side of the secondary winding 2I, which is included in an input or grid circuit 22 for each of the output tubes. It should again be noted that two tubes in balanced or push-pull relation are preferred for substantially distortionless class B" operation in an audio frequency amplifier.

The step down ratio of the coupling device or input transformer I8 for the output stage is arranged to permit a relatively low reflected impedance, over from the anode or output cir'- cuit I5 of the driver stage, in 'each half of the input or grid circuit 2'2 of the amplifier I0, in series therewith. Therefore, when the grids of the tubes I6 and I1 are driven into the positive .Is

out overloading the driver stage II and introducing distortion, and, is further of an order such that the anode circuit impedance of the driver stage reflected over into each half of the grid circuit 22 of the output stage III in series therewith, is substantially less than the grid to cathode impedance of the electric discharge device I6 or In general, these tubes are of the ordinary audio frequency amplifier type.

' In any case, the output tubes I6 and I1 are arranged to be operated in accordance with.class B operation, that is, the control electrodes or grids are biased to substantially anode or spacecurrent cut-o as indicated by the legend in the drawings, Fig. 1.

The bias source is located in the grid circuit 22, and is common to both devices I6 and I'I. In the present example, the input or grid circuit 22 for devices I6 and I'I is provided by a Winding-or impedance 2|, this being the secondary of the coupling device or transformer I8, and is mid-tapped as indicated at 24 for application of the biasing potential, in the usual manner for push-pull circuits.

The load connected with the output stage is also arranged to have a relatively low impedance with respect to the internal impedance of the output stage. To this end, the output circuit of the power stage is arranged to have a low impedance with respect to the internal or plate impedance of the electric discharge devices therein.

'I'he output circuit of the power or output stage I0 includes a balanced output or anode circuit 25 for devices I6 and II, a source of load therefor indicated by output terminals 26, and the output coupling device or transformer I9 which is interposed between the circuit 25 and the load, as an impedance transforming means.

The anode or operating potentials for the devices I6 and Il are applied through'a mid-tap connection 28 on the primary Winding 2'I of the output transformer or coupling device, and the anode circuit 25, in the usual manner for pushpull amplifiers. The plate or anode supply source which may be connected between the cathodes and the tap 28 should, as indicated, have goodv regulation for the reason that in operation the devices or tubes I6 and I1 draw a varying load current in response to changes in the impressed signal voltage.

In accordance with the invention, therefore, the grid or input circuit per se as distinguished from the entire input circuit or system, of the power amplifier or output stage, must be of low resistance and the input circuit as a whole must be of low impedance, so that the power or cur- Il in the output stage connected with that half rent requirements of the tubes through the grid circuit may be met without seriously distorting the wave shape of the signal applied to the driver stage, and the reilected load back into the output circuit, or the output circuit impedance, must be kept low with respect to the internal impedance of the output tubes in order that the tubes may be driven to the limit of their emission or space charge, without distortion in the plate circuit.

The output power is limited by the plate dissipation, the maximum directv current to the plate, or the maximum allowable grid voltage swing within the distortion limits. If .the grid voltage swing limits the output because of distortion, it does so by swinging in a positive direction until it approaches the minimum instantaneous plate voltage. When the two voltages approach each, other, the direct current of the grid rises very rapidly with any increase ln grid voltage swing. 'Ihe plate or anode current swing for a given excitation depends upon' the load resistance in the plate circuit. The load resistance, then, bears a very important relation to the tube loss, the maximum power output and the power required to drive the grid, as Will be seen hereinafter.

Referring now to Fig. 2, 29 and 30 are plate or anode-current, grid-bias curves .for tubes I and I1 respectively, of Fig. l. The curves are plotted with respect to a grid-bias scale or zero yplate current line 3l, and -an ordinate 32 representing plate current and grid current scales. The curve 29 is plotted from data obtained byv varying the applied grid potential and reading the corresponding anode current for one tube such as tube I6, in the circuit 25 of Fig. 1. The plate current curves are taken with load, that is with a load at 26 reflected over in series with one tube in the plate circuit 25.

The zero bias line for curve 29 is indicated at 33 and it will be noted that the normal anode current for class B operation is indicated at 34 through which is drawn the normal negative bias line 35. `It will also be noted that the negative bias Vis such that the point 34 on the curve 29 is substantially at zero or cut-01T for the anode current. In response to applied signal potentials, the anode current increases along the line 29 to a maximum point such as the point 36 for example. The total grid swing for such operation is indicated at 31,'

begins to draw an increasing amount of grid current indicated by the curve 40, a point of maximum slope on which is indicated at 4I.

The lowerv curve 30 for the tube Il, is the same as the upper curve 29 except that it is reversed and shifted along the bias line 3| until the curves 29 and 30 coincide as nearly as possible to a #line 42 drawn through the straight portion of the upper curve 29. The bias potential indicated atthe point 43 through which the straight line 42 passes in crossing the zero plate current line or the grid bias' line 3|, is the. proper normal negative bias potential to use for the two tubes I6 and I1 having characteristics substantially'as shown.

If the tubes or electric discharge devices are not similar, or if the anode supply voltage 'is slightly different from the values indicated by the curves shown, the bias may be adjusted until the anode current is obtained Aas indicated by the curves.

The relation of the plate or anode current curves 29 and 30 of Fig. 2 is an indication ofthe operation of devices I6 and vl1 respectively in the circuit shown in Fig. 1'. When the grid of one tube is driven in a positive direction from its normal negative bias value, the plate or anode current of. that tube increases with the swing, and flows through one-half of the primary winding of the output device I9. The output voltage for this half cycle bears substantially a linear relation to the input voltage to the amplifier. At the beginning of the next half cycle the above tube becomes idle because its grid becomes more negative and the other tube functions in a manner similar to the first except that the anode current flows in the other half of the output winding of the device I9, and the output voltage is therefore 180 degrees out of phase with the rst half Wave.

These two output waves will thus unite to form a wave similar to the input signal wave, with no distortion, if the plate current and grid voltage have a linear relation substantially as shown in Fig. 2. This relation is substantially linear as` will be seen by the similarity between the curves 29 and 30 and the straight line 42 drawn through them. v

It should be lnoted that the input coupling device or transformer I8 delivers current to the grids of the output tubes from only one side of` the secondary at any particular instant, which must be considered in the design of this device.

If the signal swings to the right, as indicated, in Fig. 2, from the zero center line or bias line 35 common for both tubes, the plate current 29 of the tube IS which may be designated as Iba, increases, and as the grid becomes positive, grid current designated as ICA, flows according to curve 40. As the signal reverses, Iba decreases and as it passes the common bias line, Iba becomes zero and plate current Iba f the other tube I1 increases according to the curve 30. Therefore, each tube functions over one-half cycle, while the other is practically idle. It should also be noted that the entire output power must be transferred from only one side of the output transformer primary during each half cycle. Therefore, the load impedance applied at 26, which either the tube I6 or the tube l1 is working into, is calculated as if only one tube is supplying the total power from one side of, transformer primary 2l, but in calculating plate dissipation, each tube functions for one-half the time so that the total plate loss is divided between the two tubes.

Referring now to Fig. 3, the alternating current component of the anode or plate current Ip, the alternating current plate voltage Ep, and the alternating signal voltage Ep, applied to the grids of the output tubes areplotted as ordinates against time, in curves 44, 45, and 46 respectively. The curve 45 for the plate voltage is shown with respect to the normal applied anode or plate poential Eb indicated by a line 41, while the cathode potential o1' zero axis is indicated by a line 48.

The normal negative grid-bias potential with respect to the cathode line 48 is indicated by a line 49, about which as an axis, the curve 46 is drawn. One-half cycle of the alternating current component of the alternating current anode or plate current Ip is shown and is drawn with respect to a line 50 corresponding to the normal anode or input circuit there is interposed between the grid circuit of the output stage and the output circuit of the driver stage, a coupling means such as a step-down transformer so designed that a reflected series impedance Rs, over from the preceding driver stage, is of a lower value than the impedance of the grid circuit between the grid and cathode of each tube in the output stage,

when said grid is at a maximum positive poten-'- tial and drawing a maximum grid current.

At the same time, in the output circuit, a coupling means such as a transformer, is provided whereby the reflected load Rp back into the plate circuit of the output stage is of a lower value than the internal impedance fp of each of the output tubes, in series with the plate circuit. f

Furthermore, the direct current resistance of the grid or input circuit of the output stage must be relatively low, together with the source of bias potential, to prevent distortion, because of voltage drop in the elements of thev circuit. Also,

- in the output circuit of the output stage, the

anode potential supply must, have good regulation.

It will be noted that a low impedance grid circuit, as a part of the input circuit for the out- `put stage, is provided without resorting to articial loading means such as a shunt resistance, in .connection with the grid circuit. 'I'he loading effect of such resistance upon the grid circuit and upon the driver stage, would seriously introduce distortion or introduce practical limits into the design, whereby the desired operating characteristics would not be obtained. Chief among the disadvantages of such a circuit would be the lowering of the applied signal voltage to the output vstage and additional load upon the driver stage.

Assuming that an applied potential ,of 2000 volts or En is provided as indicated in Fig. 3, the 1300 volts or Epm will leave an instantaneous minimum potential Ehm on the plate of 700 volts. As hereinbefore explained, this last named voltage may be and preferably is, the minimum an. ode potential necessary to maintain a maximum v flow of plate current as determined by the space charge of the particular tube. At the same time, the impedance of the output circuit should be and preferably is such that a maximum plate current Ipm may ow with minimum instantaneous applied potential. It will thus be seen, as hereinbefore pointed out, that the power output is affected bysthe load impedance and by the.

` sipation may be reached and the minimum inaocaaoo stantaneous plate voltage Ehm, Fig. 3, is sumciently high to prevent anode or space current limitation, that is, to permit Ipm to flow against the space charge and also to permit the plate current to reach values near the limit of the emission of the tubes. 4

By way of example, it has been found that it is possible to use a one k. w. output tube with class C operation for a one k. w. broadcast transmitter, and modulate the plate circuit to 100 per cent by two 350 watt tubes as a class B audio vfrequency amplifier of this type, whereas otherwise with existing systems it is necessary to use a 4 k. w. tube operating as a class B" high frequency amplifier for a one k. w. station with 100 percent modulation, with its associated input system. A broadcast transmitter having a modulator system of this character embodying the invention, is shown in Fig. 5, to which attention is now directed.

Referring to Fig. 4, a radio frequency power amplifier of the class C type is indicated at 64 and includes a plurality of electric discharge amplifying devices 65 connected in parallel with a ytuned radio frequency input circuit 66, through a coupling device '61 and radio frequency inputv terminals 68. The output or anode circuit indicated at 69 is also parallel connectedwith the devices 65 and is coupled with a radiating system`10 through a suitable output coupling device 1i.

Audio frequency modulating signals are applied to the anode circuit 68 to modulate it, through a modulator 12, which includes a driver stage 13 and an output stage 14 cascade connected, between audio frequency input terminals 15 and the anode circuit 69.

The audio frequency input terminals 15 are suitably coupled to an inputcircuit 16 for the driver stage 13 by an audio frequency coupling device or transformer 11. Between the output circuit of the driver stage indicated at 18 and the input or grid circuit of the output stage indicated at 19, a second audio frequency coupling device 80 is interposed and is designed to provide an impedance relation between circuits 18 and 19 substantially like that provided by device i8 in Fig. 1, and for the same purpose.

Likewise between the output circuit of output stage 14, as indicated at 88, and the anode circuit 69, an output coupling device 8| is connected to provide a load impedance arrangement in the output circuit 88 substantially like that provided by device I9 in Fig. 1 and for the same purpose. A radio frequency choke coil 82 and a suitable by-pass condenser 83 serves as a filter to isolate the anode circuit 69 from the modulator 12.

It will be noted that in theoutput stage 14, the electric discharge devices indicated at 84 are arranged in balanced relation between the input coupling device 80 and the output coupling device `8| in substantially the same manner as in the basic circuit shown in Fig. 1 in connection with devices I6 and i1. includes two devices 85 arranged also in balanced relation as distinguished from-the single driver tube utilized in Fig. 1.

For greater power output, thisarrangement in the driver stage has the advantage that the generated voltage from plate to plate in the output circuit thereof is doubled for a given input voltage between grid and cathode on the output stage, and the step-down ratio of the input coupling device 80 may then be increased by two, or an impedance ratio of olirit the The driver stage, however,

time the plate resistance of the driver stage in series with the primary or coupling device 80 is only increased by two. Therefore, the net gain may be one-half of the effective resistance in series with the grid, such for example as resistance Rr of Fig. 4.

In a modulator and a class C output amplifier embodying the circuit shown in Fig'. 4, tubes known as radiotrons type UV-211 have successfully been employed in the driver stage at 85 with tubes of the type UV-851 used at 84 in the class .B modulator stage i4, while in the RF power amplifier 64, tubes of the UV-204-A type have been used at 66. These tubes are designated only by way of example, since their characteristics are well known and thereby the values employed in the design of the Vmodulator may more readily be appreciated and understood.

With electric discharge devices of the types named, the coupling device 80 is preferably a step-down transformer having a turn ratio of substantially 5 to l, or l0 to l, to each side of the secondary. This provides an impedance ratio -of substantially 100 to i and with an internal plate impedance Rp in the driver stage of substantially '7000 ohms, thus providing a reflected impedance over through the coupling device 80, in series with the grid circuit 18, of substantially '70 ohms. As compared wth the internal impedance Rg of the devices 84 in the output stage, of 800 to 1200 ohms, this impedance is relatively low, as will be seen. The impedance of 800 to 1200 ohms, of course, is measured at a maximum positive potential on the grid which in the present example was substantially 80 volts.

In the output circuit 88 of the class B stage 14, the calculated load over from the load circuit 69 is provided by the power amplifier 64 and is substantially 2200 ohms. With an internal im pedance of each of devices 84 or rp of substantially 1400 ohnisvthe coupling or output device 8| is so designed that a turn ratio of one to 1.4 from each side of primary to secondary is provided, and an impedance ratio of one to two. The reiiected impedance, Rp, of the load, over into each half of the output circut 80 isthen approximately 1100 ohms, which, as will be seen, is lower than the internal impedance of the output tube associated therewith.

Witha modulator of the character described, connected as shown, an output of approximately 1500 watts is obtainable from the two tubes, which is more than sucient to modulate to 100 per cent the 1,000 watt station, provided by the remainder of the equipment including the RF amplifier indicated at 64. Itmay be noted that in the modulator shown, the grids of the devices 84 have in practice been driven into the positive range to such an extent that relatively high values of grid current were obtained. For the tubes indicated, this grid current at substantially 80 volts positive, on a positive signal swing reaches as high a value as |40 milliamperes. As' this current has to be supplied without an appreciable drop in the vpower output from the modulator above described usual class A amplier, each tube being valued at several hundred dollars. It will, therefore, be appreciated that in the use of an improved class B amplifier in a modulator such as that shown and described, a material saving in cost may be effected.

In order to simplify the diagram, the filament or cathode supply circuits have been omitted. The source of cathode current supply, however, is indicated at 86 and is a generator driven by a suitable motor 81. The positive terminal of the generator 86 should be understood as being connected to all of the cathodes, although as above mentioned, the connection lead is omitted. `The opposite or negative terminal of the generator is connected thru a common grounded supply lead 90 to all of the cathodes as indicated, and serves as a grounded cathode return lead for all circuits.

In practice this connection is provided by making allconnections to ground.

Anode or plate potentials forthe amplifier tubes and the class B modulator tubes 84 are supplied by a suitable high voltage direct current generator 9| thru a positive supply lead 92 having branches 93 and 94, in each of which branches is located a suitable ammeter 95 and 96 respectively. It will be seen from the diagram that the branch 94 supplies the anode circuit 69 in series with the output device 8|, while the branch circuit 93 supplies the class B modulator tubes 84 thru the usual push-pull circuit arrangement.

A generator 91 in series with generator 9| serves to raise the voltage supplied by the generator 9| to an initial value from which the required potentialA for operating the above named modulator and amplifier is raised by the generator 9 Generators 9| and 91 are connected in series by a lead 90 and generator 91 is'in turn connected with the common ground lead 90 as indicated at 99,'whereby the operating anode potentials are applied between the cathode and anodes of the tubes supplied by said generators.

A direct current generator, or generators, are employed in the system of the present example for the reason that they provide good regulation under a varying load condition imposed by the operation of the class B ampliiier 14. An anode supply source of this character also has a large power capacity.

In the supply of anode potentials to amplifiers of the above character, it will be appreciated that rectified alternating current may be employed. However, certain precautions must be observed to provide good regulation. An alternating current supply means of this character will be described hereinafter.

The generators 9| and 91 are also driven by a suitable motor |00, and on the same shaft therewith is indicated a third generator |0|' which at its positive terminal is connected with the common grounded cathode return lead 90 and which at its negative terminal is connected thru a circuit lead |02 to branch supply leads |03 and |04 to supply a negative biasing potential to the class "13 modulator stage 14 and to the radio frequency amplifier 64. The branch lead |04 is suitably by-passed to the ground lead 90 by a by-pass condenser |05, while in the branch lead |08 for the class B modulator, a filter and voltage reducing means isinterposed, comprising a series connected resistor |06, a illter choke coil |01 and a by-pass capacitor |08.

The arrangement is such that the grid circuit 19 of the class B modulator 14 is isolated from the bias supply |0| by the filter means and the biasing potential is `reduced to a desired value. It will be noted that one side of the filter means is connected with the cathode return lead 90 thru a return lead |09 and that between the leads |03 and |09, adjacent to the iilter; there is connected a bias voltage supply battery ||0. This last is preferably a storage battery or other suitable means for the purpose of providing a stabilizing reservoir of extremely low resistance in the grid circuit return, or in the grid circuit of the class B modulator 14. To exactly balance the anode currents of tubes 84, an additional bias adjusting battery or bias means is provided in connection with one tube. In the present example this is a battery 89 in the grid circuit of one tube.

As hereinbefore pointed out, it is essential that the grid circuit be of relatively low resistance and this also includes the source of bias potential, whereby variations in the amount of grid current drawn by the tubes 84 may have no appreciable effect upon the applied signal potentials to cause distortion. This precaution in the other portions of the transmitter is not as essentialfhence in the present example a battery reservoir is utilized only in the position shown.

In the driver stage 13 any suitable'bias supply means may be employed, and in the present exrying the current drawn by the grid circuit 19.

It is of interest to note that in the circuit shown, the direction of grid current flow, as will be well understood, is in such a direction that it charges the battery ||0, tending to maintain it in a charged condition.'

The anode or plate potential supply for the driver stage 13 is supplied by the generator 91 thru a tap connection lead |2 between it and the generator 9|. A suitable choke coil ||3 is interposed in the connection lead I2 which, together with a by-pass condenser ||4, provides a filter for the supply of anode potential to the driver stage 13. The anode potential or current supply lead 93 in the modulator stage 14 is also suitably by-passed by a condenser ||5. In each case the by-pass connection is to the common grounded cathode supply lead 90.

It will be noted that ammeters 9B and 95 are respectively included in the plate circuit of the applied to the amplifier 64, since the plate current increases proportionately with an increase in the value of the applied signals.

The plate potential applied to the class B amplier 14 and to the output-amplifier 64 through the supply lead 92 is approximately 2000 volts as delivered by the generators 9| and 91 in series. The potential applied to the plate circuit of the driver stage 13 through the lead ||2 is approximately 1000 volts.

The lament potential supplied by the generator 86 is approximately 15 volts. The biasing potential applied to the class C output amplier 64 by the generator IUI is approximately 115 volts.-

The grid bias employed in connection with the class "B amplifier stage 14, as provided by the battery I i0 and the generator |0| is substantially volts negative, the voltage being reduced from that provided by the generator by the series resistor |06. The biasing potential supplied by the source i 5| is that required to operate the devices as normal class A amplifiers.

The class C amplifier 64 is provided with the usual oscillator circuit in connection with the tuned circuit 66, having a grid leak and condenser combination, H6, in the grid lead and a suitable neutralizing condenser, |81, connected with the plate circuit 69. It will be noted that in each of the individual plate leads there is connected a resistor H5. These are for the purpose of damping out parasitic oscillations in connection with the multiple connected tubes.

The operation isas follows: With the electric discharge devices energized by operation of the supply generators, radio frequency signals from a suitable source are supplied to the terminal 68 of the class C radio frequency amplifier 64 and are transmitted through the radiating system 16 as a carrier wave. The value of the signal strength may be measured by a suitable meter H9 in the radiating system.

Audio frequency signals for modulation are applied to the terminals 15 of the modulator 12 and are amplified in the successive stages 13 and 14. The amplied output is supplied to modulate the plate circuit 69 of the amplifier 64 through the coupling means provided by the output device 3|. Because of the operation of the devices 84 in the manner hereinbefore described, and because of the result of such operation as described in connection with Fig. 1, a large audio frequency power output is obtained.

While the anode current taken by the devices 64 is-maintained at relatively low normal and average values and while the applied anode potentials are within the normal rating of the devices 84, the power output is several times that obtainable from the same devices when used in an ordinary class A amplifier. This is for the reason that the impedance of the input circuit in circuit 19 is relatively low, whereby the devices 84 may be driven far into their positive grid bias range without distorting the input wave, At the same time, in the output circuit a load impedance reflected into the circuit 80 is such-that the-output of the devices 84 is limited only by the space charge or the emission. At the same time the impedance of the load is such that the emission limits of the devices 84 are not exceeded.

A Atransmitter of this character has the advantage that the radio frequency amplifier and the modulator may be substantially separate units as shown, and may be coupled for plate circuit modulation by an isolating coupling device such as a transformer. Furthermore, by the proper design of the input and output coupling devices of the output stage of the modulator, the

C radio frequency ampliiers are primarily plate circuit modulated and the class B amplifier of the present invention is adapted to operate in connection with various loads by the choice of a suitable impedance matching means, such as a transformer, the class B amplifier of the present invention is particularly well adapted for use in combination with the class C type of ampliiler employed in transmitters.

Referring now to Fig. 5, a transmitter having a modulation system provided by a class B modulator embodying the invention, is sho-wn in connection with an alternating current source of operating potentials.

In the transmitter shown in Fig. 5, the radio frequency system includes a crystal oscillator |45 followed by two cascade connected radio frequency ampliers |46 and |41 which drive a class C" radio frequency output amplifier |48.` 'Ihe class C amplifier, like that shown in Fig. 5, is plate circuit modulated and for this purpose is provided with a plate current connection |49 in which is located an indicating ammeter |50 and a radio frequency lter means including a choke coil |5| and a suitable by-pass condenser |52 adjacent to the amplifier.

Audio frequency modulation isapplied to the circuit |49 from a class B amplifier |53 constructed after the manner of that shown in Fig. 1, and connected to the circuit |49 after the manner oi' that shown in Fig. 5. For this purpose, the amplifier is provided with an input coupling device |54 providing a low impedance input circuit and an output coupling device |55 providing a low impedance output circuit, between which are connected in balanced relation, the class B amplier devices |56.

In common with lead |49 .the anode circuit of devices |56 receives its operating current or potential from a supply lead |51, and biasing potentials are supplied by a suitable source |58. Audio frequency signals are supplied to the input coupling device |54 from a suitable source indicated at |59. This may be a speech amplifier as indicated. Direct current anode potentials are delivered to supply lead |51 from an alternating current source indicated by terminals v |60, thru a suitable full wave rectier |6| which includes a pair of cathode rectifier devices |67.

The cathodes of the device |56 and the devices |62 are heated from the source |60 thru a separate transformer |63 which permits the cathodes to be separately heated before the anode potential supply is excited. For this purpose, a separate switch |64 is providedv for the cathode heating transformer |63 and another switch |65 is provided for energizing the rectier 6 .The .positive and negative supply leads |66 and |61, respectively, from the rectifier are connected with the cathodes of devices |56 and with the positive supply lead |51. In one of the leads, preferably in the positive supply lead |66, is connected a choke coil |68 in association with a suitable by-pass condenser |69, whereby the desired regulation is obtained under load conditions imposed by the devices |56. 'Ihe operation of the filter means provided by the choke coil |68 and by-pass condenser |58 will hereafter be considered.

The coupling device |55 should preferably be designed to couple the devices |56, which may be of the radiotron UV--203-A type, as class B audio amplifiersv for a. maximum. anode current of approximately 400 milliamperes for the type of tubes above mentioned, and operated at approximately 1000 volts. Such a loading permits approximately maximum output of the class "B amplifier. Higher currents to the class C amplifier will result in an overloading of the class B tubes |56. It, should be noted that during a peak modulation of 100 per cent the input power to the plates of the class C amplifier |48 increases 50 per cent so that the plate dissipation increases by the same percentage. However, the plate current to the class C tubes as vindicated by the meter |50, will remain steady so that care must be taken not to exceed the tube rating.

If a maximum output is desired from the modulator |53, medium power tubes should be used in the output stage of the speech amplifier |59 to excite the class B tubes |56 thru the coupling device |54.

It will be noted that a resistor |10 is eniployed in the anode circuit of each of the devices |56. These may be of a value of approximately 4,0 ohms, for example, and serve to prevent any high voltages because of high oscillations, or surges in the plate circuits of the class B tubes. The radio frequency choke coil |5| and the condenser |52 prevent radio frequency currents from entering the transformer |55. 'I'he C" battery |58 for the class B tubes, is adjusted to such a value that each tube draws a normal plate current of approximately 2O to 30 milliamperes for the type of tubes above mentioned, this being substantially plate current cut off for such tubes. The battery |58 represents any suitable source of C bias having a low resistance as described in connection with the preceding embodiment of the invention, and is preferably n. heavy duty type of fB battery.

A meter |1| is placed in circuit with the anodes of the devices |56. As the maximum plate current to a class B" audio frequency modulator or amplifier depends upon the power output, the variations of the meter |1| may be taken as a fair indication of the percentage modulation.

It will be noted that the secondary of the output coupling device |55 is arranged in two sections |12 and |13, whereby the sections may be connected in series, or in parallel as shown. The type of connection depends upon the type of tubes used in the class C radio frequency amplifier |48. For lower plate currents at higher voltages the series connection may be used, while for higher plate currents and lower voltages the parallel connection may be used.

It will be noted that the rectifier |6| or anode supply circuit of which it forms a part, is the usual type of full wave rectifier. However, the filter reactor |68 is a special design to improve regulation, and the filament transformer |63 is separate from that of the rectifier |6| so that hot cathode mercury vapor tubes may be employed at |62 and heated before the plate supply switch |65 is closed. During silent periods of operation of the transmitter, the filaments of all of the tubes may be left on while the plate supply is turned off, thereby requiring a minimum time to start when placed again in operation.

With reference to the filters |68 and |69, the

inductance |68 is preferably of the iron core type I in series, as shown, with the rectifier source, and is tuned as by the condenser |69 to a frequency such as the second harmonic being 120 cycles for a 60 cycle supply circuit. An indicated in the drawings, there should be no filtering b etween the rectifier and the resonant filtering element because it is arranged to operate upon an appreciable alternating current component. Any additional filter means, such as a shunt condenser |14, may be added in lthe anode supply circuit following the filter shown, altho none is necessary in the supply circuit of the present example.

The circuit arrangement is such.that in connection with the filter, an increase in the current taken by the load or by the class B amplifier |53 in the present example, de-tunes the tuned filter element and results in a tendency to raise the voltage across the output of the supply source because of a drop in the impedance to the unfiltered alternating current component. The 120 vcycle component'is selected as the strongest of the available harmonics of the supply source.

It is preferable also that the choke coil |68 should be so designed that normally sufiicient current may flow thru the choke coil to bring it near to its saturating point, since a change of reactance for regulation purposes must be proper-@sii frequency which reduces the output voltage to a minimum.

The choke coil is also designed so that it tends to saturate as the load current is increased so that it tends to introduce a lower reactance in the circuit for greater load currents, while at the same time the change in reactance de-tunes the circuit, which also tends to sharply reduce the impedance of the circuit to maintain the tube voltage more nearly constant regardless of the tube current within the power range of the plate supply means. The `condenser |14 functions as a normal filter condenser and should be relatively large to effectively by-pass the a. c. components to currents to the class B modulator.

While the invention has been illustrated and described for purposes of convenience in its present application to audio frequency amplifiers generally, and in systems for the modulation of high ,power transmitters, it is obvious that it is not limited thereto, but may be applied to any elecf trical apparatus requiring large quantities of fluctuating electric power, without involving costly and complicated equipment including a large number of amplifier devices and associated potential supply apparatus.

I claim as my invention:

1. The combination with a radio frequency vacuum tube amplifier, having an anode circuit, of means for supplying audio frequency power to said anode circuit, said means including an audio frequency amplifier having a balanced audio frequency input circuitand a balanced audio frequency output circuit, an impedance changing device interposed between said last named circuit and the first named anode circuit as a coupling device, the coupling ratio being such that the outaoeaaao put circuit operating impedance is less than the internal impedance of said amplifier, and the input circuithaving an impedance which is lower than the lowest input operating impedance of said amplifier, whereby the power output may be proportional to the square of the input potential.

2. The combination with a radio frequency power amplifier, having an anode circuit, of means for supplying audio frequency power to said anode circuit, said means including an audio frequency amplifier having a balancedaudio frequency input circuit and a balanced audiol frequency output circuit, an impedance changing device interposed between said last named circuit and the first named anode circuit as a coupling device, the coupling ratio being such that the output circuit alternating current impedance is less than the internal impedance of said amplifier, and the input circuit having an alternating current impedance which is lower than the lowest input operating impedance of said amplifier when drawing a predetermined maximum grid current, whereby the power output may be proportional to the square of the input potential.

3. In a radio transmitting system, vthe combination with a radio frequency output amplifier of the type having a power output which varies as the square of the anode potential within predetermined limits, and an anode circuit therefor, of an audio frequency amplifier means for applying audio frequency power to said anode circuit, said means includingv a pair of electric discharge devices each having an anode, a cathode, and a control grid, said devices being connectedin balanced relation to each other and being arranged to provide a power output proportional to the square of the excitation grid voltage, an input circuit of low impedance with respect to the grid impedance of each of said devices when drawing a predetermined maximum grid current, an output circuit of low impedance with respect to the anode impedance of each of said devices, said output circuit including an output coupling device connected with said rst named anode circuit, and means for. applying audi`o frequency signal potentials to said input circuit.

4. In a radio transmitting system, the combination with a radio frequency output amplfer of the type having a power output which varies as the square of the anode potential within predetermined limits, and an anode circuit therefor, of an audio frequency amplifier means for applying audio frequency power to said anode circuit, said means including a pair of electric discharge devices each having an anode, a cathode, and a control grid, said devices being connected in balanced relation to each other and being arranged to provide a power output proportional to the square of the excitation grid voltage, an input circuit of low alternating current impedance with respect to the grid impedance of each of said devices when drawing a predetermined maximum grid current, an output circuit of low alternating current impedance with respect to the anode impedance of each of said devices, said output circuit including an output coupling device connected with said rst named anode circuit, and means for applying audio frequency signal potentials to said input circuit.

5. Means for plate modulating a class C radio frequency amplifier comprising an audio frequency amplifier transformer coupled to the plate circuit of said first named amplifier, a radio frequency lter means interposed in said plate circuit between said transformer and said class C amplifier, and a speech amplifier coupled to the input circuit of said class B audio frequency amplifier, said first named amplifier having a balanced audio frequency input circuit and a balanced audio frequency output circuit, and having input and output circuits provided in connection with said transformer coupling and said speech amplifier coupling the operating impedancesofV which input and output circuits are less, respectively, than the lowest internal operating input and output impedances of said devices when drawing a predetermined maximum grid current.

6. Means for plate modulating a class C radio frequency amplifier comprising a class B audio frequency amplifier transformer coupled to the plate circuit of said first named amplifier, a radio frequency filter means interposed in said plate circuit between said transformer andv said class C amplifier, a speech amplier, and means for coupling said class B audiofrequency amplifier to the speech amplifier comprising a stepdown coupling transformer providing in the input circuit of said class B amplifier an alternating current impedance lower than the lowest input impedance of the amplifier when drawing maximum grid current, and an output coupling transformer for the class B. audio frequency amplifier having impedance changing windings adapted for series and parallel connection in the plate circuit of the class C amplifier.

7. A modulator for a radio frequency amplier comprising a push-pull class A amplifier stage,

a balanced second stage amplifier coupled to said class A amplifier stage, of the class B audio frequency type comprising a pair of balanced electric discharge amplifier devices and including a stepdown input transformer and a step-down output transformer, the coupling ratios of said transformers and the impedances in circuit therewith being such that the output circuit operating impedance is less than the rinternal impedance of said second stage amplifier, and that the input circuit impedance is lower than the lowest input operating impedance of said second stage amplifier, means for supplying modulation signals t the input circuit of said first stage amplifier, and a. radio frequency amplifler having an anode circuit coupled to the output of the class B stage through said output transformer.

A 8. A modulator for a radio frequency amplifier comprising a push-pull class A amplifier stage, a balanced second stage amplifier of the class B audio frequency type comprising a pair of balanced electric discharge amplifier devices and including Aa. step-down input transformer and a step-down output transformer, the coupling ratios of said transformers and the impedances in circuit therewith being such that the output circuit operating impedance is less than the internal impedance of said second stage amplifier, and that the input circuit impedance is lower than the lowestinput operating impedance of said second stage amplifier, and means for supplying modulation signals to the input circuit of said first stage amplifier, and said output stage including a grid circuit through which grid current flows in response to signal currents and a storage battery connected in said grid 'circuit to supply substantially constant grid potentials thereto and to be charged by the flow of grid current.

9. In a radio transmitting system, the combination of a radio frequency amplier comprising a plurality of parallel connected electric discharge amplifier devices having a grid circuit and an anode circuit, means for supplying radio frequency signals to said grid circuit, means for coupling said anode circuit to radiate signals, said grid and anode circuits being tuned, a modulation input transformer connected in the low potential side of the anode circuit of said first named amplifier. a radio frequency mter interposed between said transformer and said first lnamed amplifier in the anode cirouitlthereof. a balanced modulation signal electric discharge amplier connected with said transformer, said amplifier being an audio frequency power amplier comprising a pair of electric discharge amplifier devices having balanced low impedance input and output circuits, the impedances of Vwhich circuits are lower than the lowest corresponding input and output impedances of said amplifier devices when drawing a maximum predetermined grid current and said devices being arranged to operate alternately, a balanced electric discharge driver stage for said amplifier and means for supplying modulation signals to said driver stage.

10. Means for plate modulating a class C radio frequency amplifier comprising a class 'B audio frequency amplifier transformer coupled to the plate circuit of said first named amplifier, a radio frequency filter means interposed in said plate circuit between said transformer and said class C amplifier, a speech ampliiler, and means for coupling said class B audio frequency amplifier to the speech amplifier comprising a step-down coupling transformer, said class B amplifier being of the audio frequency type comprising a pair of electric discharge amplifier devices having balanced input and output circuitsthe operating impedances of which are lower at all times than the lowest corresponding input and output impedances of said devices when drawing a. maximum predetermined grid current and said devices further being arranged to operate uiternately.

` LOY E. BARTGN; 

